The art is replete with prior uses of sonar ranging equipment carried by manned surface or submarine vessels for underwater detection of moving and stationary underwater targets or objects, including depth and fish finders and the like, wherein the acoustic pulse or other transmitter source is co-located adjacent an onboard acoustic receiver and its target range displays of the received echoes from such targets.
An example of such systems with co-located sound source and receiver for determining the position of a target vehicle (range and azimuth angle) is shown in later-discussed FIG. 3b. This method is used by many of the active sonar systems in the navies of the world, as described, for example, in Principles of Underwater Sound, Urick, Robert J., McGraw Hill Book Company (1975), and also in commercial side-scan sonar systems, and the like. In this method, a sound source is co-located with the receiver system (typically the receiver is a hydrophone array). A specialized signal is transmitted into the water. The received signals are filtered so that the reflections from underwater objects are amplified and noise is reduced. The delay between the source transmission time and the time at which the reflected signal is received by the array is combined with the known sound speed to determine the range to the vehicle. The received signals are also processed to determine the azimuth angle or bearing to the vehicle. The method uses the time delay between the (synchronized) sound source transmission and the received reflection (along with the known sound speed) to determine range; however, the sound source is co-located with the receiver and not on board the target vehicle, a different configuration from the present invention. This method may also be used with the sound source and the receiver array located in different positions.
The present invention, however, is not concerned with such ranging configuration systems, but rather with very different configurations wherein the acoustic transmitter is carried by unmanned (or autonomous) UUV or AUV type underwater vehicles, acoustically interactive with remotely positioned hydrophone or similar acoustic receivers (all hereinafter generically referred to as “hydrophones”) that have been deployed, as on sonobuoys and the like, in predetermined patterns and remotely from the vehicles.
Other types of systems than the present invention, however, have also employed predetermined patterns of hydrophone receivers for determining the position (x-y-z coordinates) of a vehicle operating in an underwater acoustic range, as described, for example, in Gunderson, Charles R., “The History of the Naval Torpedo Ranges at Keyport,” NUWC Div. Keyport Report No. 2254, Keyport, Wash., August 1998. These methods, also later described in connection with FIG. 3c, are of two types; long baseline (LBL) and short baseline (SBL), depending on the particular geometry of the hydrophone receivers and the resulting equations used to compute the vehicle position. They are of two classes; synchronous source (SS) and asynchronous source (AS), depending upon whether the sound source is synchronized with the receiver processing system or not. For such acoustic test facilities, the precise locations of the fixed hydrophone system must be known, a-priori. The receiver system consists of several hydrophones placed at fixed locations (a minimum of 2-4 hydrophones are needed, depending on the method used), typically along the sea floor. Signals from these hydrophones are recorded and processed by the appropriate systems. A sound source (sometimes called a projector) is placed in the vehicle and, depending on whether the SS or AS method is to be used, may need to be synchronized with the receiver system before deployment of the vehicle. The projector periodically transmits or emits a special signal that is detected by the hydrophone system. The hydrophone processing system computes the time at which the signal arrives at each hydrophone. In the SS method, range is computed directly from the time delay between the known source transmission time and the arrival time. In the AS method, range is inferred from the differences in the signal arrival times at several hydrophones in predetermined locations.
Such test range AS methods, however, are different from the present invention in that: the source in the vehicle is not synchronized with the receiver system, whereas the invention uses a synchronized source. The receiver hydrophones, moreover, must be fixed at precisely known positions, whereas the invention may use hydrophones or hydrophone array receivers attached to drifting buoys or moving vehicles containing GPS receivers. The AS technique is only applied to determine the vehicle location, whereas the invention uses the projector signal to make additional acoustic measurements (TL, bottom loss, sonar calibrations, etc) and/or for target training.
While the SS methods may use a source contained in the vehicle that is synchronized with the receiver system before deployment, they are not adapted for or used with drifting buoys or moving vehicles containing GPS receivers, nor do they make such additional acoustic measurements.
This is further described in Cecil, Jack B., “Underwater Hydrophone Location Survey”, Proceedings of the Precise Time Interval Systems and Applications Meeting, Vol. 24, No. 15, 1992. The synchronized sound source was suspended from a surface ship whose position was monitored via radar. The time delay between the source transmission and the received signals was used to determine the unknown range to a set of bottom-mounted hydrophone receivers. While the range calculation was similar to that used in the present invention, the source was located in a fixed surface ship and not a mobile underwater vehicle, with its different operational characteristics.
Another approach for tracking underwater vehicles is known as the GPS Intelligent Buoy (GIB) system of U.S. Pat. No. 5,579,285, also described in                Bechaz, C. and Thomas, H., “GIB Portable Tracking Systems—the Underwater Use of GPS,” Hydro International, 2002;        Thomas, H., “GIB buoys: An interface between space and depths of the oceans,” in Proceedings of IEEE Autonomous Underwater Vehicles, Cambridge, Mass., USA, pages 181-184, Aug. 1998; ORCA ACSA. Trajectographe GIB Manuel Utilisateur, 1999.        
Such is represented in later-discussed FIG. 3c, using a GPS time-synchronized source, attached to the underwater vehicle, to produce high frequency pings (8 to 50 kHz) at precise intervals. The pings are received by a set of hydrophones attached to buoys on the water surface (usually four) that are also equipped with GPS systems to locate the buoys and provide an accurate clock. The acoustic propagation time delays are measured to each buoy from the source vehicle. The ranges from each buoy to the vehicle are estimated by assuming the sound speed. The vehicle position is then determined from the known buoy positions and the estimated ranges. This system also uses a second ping (transmitted after the initial ping) to provide the depth of the vehicle. The GIB system operates at high frequencies and has been used for tracking underwater objects with the externally attached synchronized pinger at ranges less than 3 km. This system is adapted to be used solely for tracking underwater objects and is not adapted to be used for determining any of the other acoustical purposes as in the present invention, such as the measurement of TL or bottom loss measurements, sonar calibrations, and/or for target training. Although only hydrophone receivers are used (no arrays), the GIB system relies on the same basic principle to determine the range to the vehicle as is used in the current invention (time delay from a synchronized source in the vehicle and an assumed sound speed). The present invention, however, is a low frequency (200 to 2,000 Hz) system that tracks the underwater vehicle to much greater ranges (ranges up to 20 and even 50 km have been demonstrated), and additionally provides simultaneous acoustic measurements and/or training capabilities with the calibrated acoustic source that is integral within the UUV and not externally attached to it.
In contrast to these prior techniques, the present invention is specifically designed also to measure important acoustical properties of the ocean. The system, as before stated, is comprised of an acoustic source mounted on an underwater AUV or UUV vehicle or the like, and hydrophone receivers, deployed in the ocean, as by floating buoys, and separated or remote from the vehicle. Prior to launch, the vehicle is programmed to: i) run a series of defined tracks (e.g. opening, closing, circling, changing speed and/or depth) and ii) transmit specialized acoustical waveforms that, upon signal processing, provide range from the vehicle to the receiver and other acoustical properties (e.g. transmission loss or the channel impulse response). The vehicle and receivers have internal clocks that are time-synchronized. After the source transmits the waveforms, the time delay to the receiver is measured by utilizing matched-filter processing. The range from the source is determined by the product of the sound velocity and the measured time delay. This process is continually periodically repeated, preferably once per minute, and the range is measured in close to real time throughout the duration of the system run plan.
Details as to the AUV or UUV programming and operation are presented, for example, in U.S. Pat. Nos. 5,666,900; 5,600,087; 5,537,947; 5,490,573; and 5,487,350.